1. Field of Invention
This invention relates to an apparatus for measuring combustible gas concentration in a flue gas occuring in combustion processes; and more particularly, to such an apparatus which is capable of sensitively measuring the concentration of a very small amount of combustible gas in a flue gas which is in an O.sub.2 gas rich state.
2. Discussion of the Prior Art
In a combustion process, it is preferable to monitor at all times the concentrations of O.sub.2 and a combustible gas such as CO in the flue gas flowing through a duct or stack and effect combustion control so that the combustion furnace is run in optimal conditions from the viewpoint of conservation of energy and prevention of environmental pollution. In general, the optimal value of O.sub.2 is within the range of from 1 to 5%, while the optimal value of CO is within the range of from 150 to 300 ppm. Therefore, the flue gas is in a state wherein O.sub.2 gas is rich.
As one example of apparatus for measuring CO gas in the above state, a catalytic combustible sensor is commercially available from AMETEK, Inc and called "WDG-IIIC". In this type of sensor, the flue gas, which is sucked in from the duct or stack through a filter, is guided outside of the duct through a pipe to a sensor section where it is burned catalytically, and the concentration of combustible gas contained in the flue gas is measured on the basis of change in the temperature.
In this type of prior art measuring system, however, the sensor cannot be inserted directly into the duct and hence there is a need for a suction mechanism for guiding the gas outside of the duct through a pipe. Furthermore, the gas to be measured must be supplied to the sensor section at a predetermined flow rate. However, the flow rate of the suction gas may be changed by clogging of the piping outside of the duct, resulting disadvantageously in a zero drift or span drift. The prior art further suffers from the disadvantage that the sensor sensitivity is low at a combustible gas concentration of about 200 ppm which is essential to combustion control.
Another prior art apparatus for measuring CO gas in an O.sub.2 rich state comprises an infrared CO meter and is commercially available from Land Combustion, Ltd. In this prior art apparatus, a light emitting section, which emits infrared light that contains a wavelength region that is absorbed by combustible gas, and a light receiving section, which receives the infrared ray which has been subjected to absorption, are provided directly at both sides of the duct to measure the combustible gas concentration. This type of measuring apparatus has the advantage that it is possible to selectively measure the concentration of a desired combustible gas even if a plurality of combustible gases are mixed together and contained in the flue gas. Also, the response is quick as compared with the catalytic combustible sensor. Moreover, the sensitivity is high at the essential combustible gas concentration.
This prior art infrared CO meter has many disadvantages. For example, since this prior art apparatus is an optical system, the mechanism is complicated and the overall size thereof is increased, which results in an increase in the cost. Since this apparatus cannot be installed in a place where the temperature is relatively high, the installation conditions are limited. After the apparatus has been mounted on the wall of a duct, no calibration can be effected using a standard gas. Therefore, the reliability of such prior art apparatus is disadvantageously low.
U.S. Pat. No. 4,231,733 discloses a sensor which is capable of separately measuring the concentration of O.sub.2 gas and a combustible gas in a flue gas by use of an oxygen ion conductive solid electrolytic cell. In this sensor, when O.sub.2 in the gas which is to be measured is O.sub.2 rich, measurement is carried out in a voltage measuring mode to thereby meassure the O.sub.2 gas concentration on the basis of the level of the electromotive force which changes in accordance with the oxygen partial pressure difference between the measured gas and a reference gas. When there is very little O.sub.2 in the measured gas and/or the combustible gas is rich, the measuring circuit is switched over so as to connect a constant voltage source between two electrodes of the sensor to measure the amount of current flowing in accordance with the amount of oxygen consumed by the combustion reaction of the combustible gas, thereby measuring the combustible gas concentration.
However, the conditions in which the combustible gas can be measured with this system are such that the O.sub.2 concentration is 0.01% or less and the combustible gas is rich. On the other hand, in the combustion control the measurement of the combustible gas concentration is carried out with respect to the flue gas which is in a state wherein the concentration is from 1 to 5%, while the amount of CO gas which is to be measured is very small, i.e. from 150 to 300 ppm. Thus, this prior art apparatus cannot be used for such purposes.
Japan Laid Open Patent No. 60-61654 discloses an apparatus which is capable of measuring a very small amount of combustible gas in a flue gas which is in an O.sub.2 gas rich state, by the use of a solid electrolytic cell. In this apparatus, a measuring electrode which is contacted by a gas to be measured is provided at one side of a solid electrolyte, while a reference elettrode which is contacted by a reference gas whose oxygen partial pressure is khown, is provided at the other side of the solid electrolyte, to thereby constitute a measuring cell. The measuring electrode comprises a first electrode which allows combustion, i.e. catalytic, reaction to proceed until the O.sub.2 gas and combustible gas in the gas to be measured reach a chemical equilibrium, and a second electrode having relatively low catalytic activity which prevents the combustion reaction from proceeding until the O.sub.2 gas and combustible gas in the gas to be measured reach a chemical equilibrium. Calculation is made on the basis of two different kinds of electro motive forces which are respectively generated between the first and second measuring electrodes and the reference electrode, thereby enabling measurement of the respective rpartial pressures of the combustible gas and the O.sub.2 gas in the measured gas. Moreover, this apparatus uses for the second electrode material having relatively low catalytic activity, such as platinum material, such as platinum, platinum-vanadium alloy, or the like, and gold material, such as gold, gold-platinum alloy, or the like.
However, when a platinum material is used for the second electrode, the catalytic power is so high that a very small amount of combustible gas cannot be measured with sensitivity. When a gold material is used, gold diffuses into the solid electrolyte as time passes, thus causing detecting characteristics to become deteriorated.
When the first and second measuring electrodes are heated, they are disposed in an enclosed heating space. The first electrode which has a high catalytic activity causes the combustible gas and O.sub.2 gas in the gas to be measured, to react actively with each other, so that the combustible gas is oxidized. Since the gas which is measured can reach the electrode portion only by diffusion, the concentration of the combustible gas around the second electrode changes, and this affects the results of the measurement.
Furthermore, since the first and second electrodes are heated at the same temperature, it is impossible to select an optimal temperature for each of the electrodes. Thus, each electrode is unable to conduct measurement under optimal conditions.